Xolile Mngeni, 25, was found guilty of robbery with aggravating circumstances, premeditated murder and illegal possession of a firearm by the Western Cape High Court, SAPA news agency reported.
His co-accused have said they shot dead Anni in November 13, 2010 while she was on honeymoon in Cape Town on orders of her NRI husband Shrien Dewani.
But Shrien, a British businessman, denies plotting to kill his wife and is still fighting extradition to South Africa.
"The State has proven its case beyond reasonable doubt," Judge Robert Henney said in his verdict after the trial, which lasted three-month.
Mngeni looked stony-faced throughout the judgement.
He denied the charges but Judge Henney said the accused's "evidence was riddled with inconsistencies and untruths".
Mngeni's trial had been repeatedly delayed while he had surgery to remove a brain tumour, but a judge ruled he was fit to face justice.
The case has received massive public attention in both the UK and South Africa.
Taxi driver Zola Tongo says he was approached by Shrien, who offered him about USD 2,100 to organise the killing and make it look like a carjacking. He was sentenced to 18 years in prison following a plea bargain.
In August, Mziwamadoda Qwabe was sentenced to 25 years in prison after pleading guilty to shooting Anni.
